1. Field of the Invention
The present invention relates to a photothermographic imaging material, and particularly to a photothermographic imaging material with high density which is excellent in light radiated image stability, silver color tone and the like, and to a method for forming an image by using the same.
2. Description of Related Art
Recently, in the fields of medical care and print plate making, waste solutions involved in wet processings of image formation materials have been problematic in terms of working property, and reduction of processing waste solutions has been strongly desired in the light of environmental preservation and saving space. Thus, technology concerning photothermal photographic materials for photographic technology use such as laser imagers and laser image setters where efficient exposure is possible and clear black images with high resolution can be formed has been required.
As the technology according to the above photothermal photographic materials, for example, known are silver salt photothermographic dry imaging materials (hereinafter, also referred to as photothermographic imaging materials or simply imaging materials) containing an organic silver salt, photosensitive silver halide and a reducing agent on a support (e.g., U.S. Pat. No. 3,152,904 specification, U.S. Pat. No. 3,487,075 specification, D. H. Klosterboer, “Dry Silver Photographic Materials”, (Handbook of Imaging Materials, page 48, 1991, Marcel Dekker Inc.)). This silver salt photothermographic dry imaging material has an advantage capable of providing users with a system which is simpler and does not impair the environment because no solution type processing chemical is used at all.
This photothermographic material is processed by a thermal development apparatus which adds stable heat to the photothermographic material to form the image, typically called a thermal developing apparatus. As mentioned above, in conjunction with the recent rapid prevalence, this thermal developing apparatus has been supplied in the market in large quantities. In the meanwhile, there has been problematic in that slipping property between the imaging material and a transport roller or processing members of the thermal developing apparatus changes, and transport failure and density unevenness occur. Also there has been problematic in that the density of the photothermographic imaging material varies with time. It has been found that these phenomena noticeably occur in the photothermographic imaging materials where image exposure is performed by laser light and subsequently the image is formed by thermal development.
Also recently, downsizing of laser imagers and acceleration of processings have been required. Therefore property improvement of the photothermographic imaging materials becomes essential. For downsizing the thermal development processing apparatus, it is more advantageous to use a heat drum mode than to use a horizontal transport mode, but there has been problematic in that powder drop off, density unevenness and roller mark easily occur at the thermal development processing. Also, even when the rapid processing is carried out, to obtain sufficient density of the photothermographic imaging material, it is effective to enhance covering power by increasing coloring point numbers using silver halide with smaller average particle size as shown in JP-A-11-295844 and JP-A-11-352627, to use reducing agents with high activity having secondary or tertiary alkyl groups (see JP-A-2001-209145), and to use development accelerators such as hydrazine compounds and vinyl compounds.
However, when these technologies were used, there was problematic in that density changes (printout property) with time after the thermal development processing became large and the silver color tone became extremely different (took on a yellow tinge) compared to wet type X-ray films in earlier technology. Additionally, a new problem where the color tone takes on a red tinge at high density areas with density of 2.0 or more has occurred when those with smaller average particle size are used as the silver halide.
On the other hand, in image diagnosis by imaging materials for the medical use, silver color tone formed by development is an important factor which determines good or poor image quality. A silver ion reducing agent, a compound which forms a complex with the silver ions, a compound which bleaches fine silver nuclei which become sources of photographic fog which produces on surfaces of silver halide grains, and the like are contained in the silver salt photothermographic dry imaging material, and thus it is not easy to control developed silver shapes and retain images after the thermal development. That is, color tone changes must be reduced not only immediately after the thermal development of the imaging material but also in a long term storage before the thermal development and in image storage after the development. For example, disclosed is the method for reducing the ingredient having reducibility contained in the silver salt photothermographic dry imaging material (e.g., see JP-A-2002-328442). However, the color tone in the image storage is improved, but the color tone immediately after the thermal development is not improved. In earlier technology, these improvements have been attempted by controlling developed silver shapes. For example, disclosed is the method where the “color tone” changes under an atmosphere with high moisture is reduced by reducing particle sizes of silver halide grains and fatty acid silver salt crystals and controlling a “potency range” at the thermal development to the certain range (e.g., see JP-A-10-282601). Also, proposed are the improvement methods by activating photothermographic property by contrivance of fatty acid silver salt crystal structures (e.g., see JP-A-2002-23303 and JP-A-2002-49119), but it can not help being said that all the methods are at insufficient levels in terms of realizing the stable silver color tone. Also disclosed is the method using leuco compounds which imagewisely produce yellow compounds by oxidation-reduction reaction at the thermal development, in combination with the certain silver ion reducing agent (e.g., see JP-A-2002-169249). However, the technology described in JP-A-2002-169249 is more excellent in improvement level of the color tone compared to the above technology which controls the developed silver shape, but has disadvantages that the photographic fog and deterioration of the color tone changes frequently occur in the long term storage and in the image storage probably because produced dyestuffs are unstable and further adversely affect the silver halide.
Also, in the light of effectively utilizing the silver which is a valuable resource, efforts to increase the maximum density on the imaging materials at an identical amount of the silver must be continued. A basic technical concept for this is to make individual developed silver small at the identical silver amount and make the particle sizes of photosensitive silver halide grains small. That is, the combination with so-called sensitization technology becomes essential. But when the individual developed silvers are made small, extents of optical scattering and absorption are changed and thus the silver color tone is changed. Further, when the chemical sensitization is given with a Te sensitizer and a gold sensitizer, the photographic fog is increased. Thus, a new technology where the increase of maximum density, sensitization, low photographic fog and color tone are compatible has been required.